Minimize Number of Requests
When a webpage is loaded the browser loads the main page file(.html, .php,…) and then it starts loading the files referenced by it: css files, js files, images,… The more requests are required, the longer it takes to load the page. There is also a limit for most of the browsers with the files that are loaded simultaneously from the same domain. Firefox can load only 2 simultaneously from the same domain.
Most of the time when a page is loaded it takes only a short time to load the actual page file. This represents for most of the pages 5%-20% of the total loading time. The rest is wasted on loading css, js, images or other types of files. In order improve the loading speed of a page you have decrease the number of requests done when a page is loaded.
Join .css and .js files – first of all you have to join all the js files under a single one and to do the same with the css files. In practice it happens to have the css and js split in several files.
Use css sprites and join the images which are used in multiple pages. Css sprites represents a technique which consist in joining multiple images under a single one, and use css to display in the right spot in the browser only the cropped version which represents the original image.
Distribute Static Content on Multiple Servers
In practice is almost impossible to put all the content in singular files. When you deal with multiple files that can not joined, you can distribute them on different servers. For example the css and js file will be loaded from static1.domain.com, some of the images will be loaded from static2.domain.com and other set of images from static3.domain.com.
The method of distributing static content on several servers has the advantage of providing the browser the option to load simultaneously the more than 2 files. The drawback is the fact that the browser should make more dns lookups to retrieve the ip addresses of the servers where the content is distributed. However, the benefits of this method are much better than the drawbacks.